1. The Field of the Invention
The present invention relates to glass solders, in particular amorphous and partially crystalline glass solders, which are particularly suitable for high-temperature applications and their uses.
2. The Description of the Related Art
Glass solders are usually used for producing joints, especially to join glass and/or ceramic components to one another or to components made of metal in an electrically insulating manner. In the development of glass solders, the composition thereof is often selected so that the coefficient of thermal expansion of the glass solder corresponds approximately to that of the components to be joined to one another in order to obtain a joint which is stable in the long term. Compared to other joints, for example those composed of plastic, those based on glass solders have the advantage that they can produce a hermetic seal and can withstand relatively high temperatures.
Glass solders are often generally produced from a glass powder which is melted during the soldering operation and together with the components to be joined forms the joint when it is heated. The soldering temperature is generally selected so as to correspond approximately to the hemisphere temperature of the glass or can deviate from the latter usually by ±20 K. The hemisphere temperature can be determined by a microscopic method using a hot stage microscope. It characterizes the temperature at which an originally cylindrical test sample has melted together to form a hemispherical mass. The hemisphere temperature can be assigned a viscosity of about log η=4.6, as can be seen from the relevant technical literature. If a crystallization-free glass in the form of a glass powder is melted and cooled again so that it solidifies, it can usually be remelted at the same melting point. In the case of a joint comprising a crystallization-free glass solder, this means that the operating temperature to which the joint can be subjected in the long term must be no higher than the soldering temperature. In actual fact, the operating temperature in many applications has to be significantly below the soldering temperature, since the viscosity of the glass solder decreases with increasing temperatures and a glass having a certain flowability can be expressed from the joint at high temperatures and/or pressures, so that the joint can fail. For this reason, glass solders for high-temperature applications usually have to have a soldering temperature or hemisphere temperature which is significantly above the future operating temperature.
One field of use for such glass solders is, for example, joints in high-temperature fuel cells which can be used as an energy source in motor vehicles or for decentralized energy supply. An important type of fuel cell is, for example, the SOFC (solid oxide fuel cell), which can have very high operating temperatures of up to about 1100° C. The joint comprising the glass solder is usually used for producing fuel cell stacks, i.e. for joining a plurality of individual fuel cells to form a stack. Such fuel cells are already known and are continually being improved. In particular, the trend in present-day fuel cell development is generally in the direction of lower operating temperatures. Some fuel cells are now able to achieve operating temperatures below 800° C., so that a lowering of the soldering temperatures is possible and also desirable because of the resulting low thermal stress on the SOFC components during the soldering process.
An important role in fuel cell development is played by glass solders which have been the subject matter of the following disclosures.
DE 19857057 C1 describes an alkali-free glass-ceramic solder having a coefficient of thermal expansion α(20-950) of 10.0×10−6 K−1 to 12.4×10−6 K−1. The solder described there contains from 20 to 50 mol % of MgO. Glasses having a high MgO content are in practice highly susceptible to crystallization, which leads to compounds which crystallize rapidly and to a high degree. In the case of such rapid and substantial crystallization, it is difficult to ensure good wetting of the material to be joined by the glass solder. However, this is necessary to be able to provide a joint which optimally satisfies the respective requirements. In addition, the glass solder described in this document contains from 40 to 50 mol % of SiO2. However, an increasing content of SiO2 leads to an increase in the melting point and thus also the soldering temperature.
Likewise glass-ceramic solders are described in U.S. Pat. No. 6,532,769 B1 and U.S. Pat. No. 6,430,966 B1. These solders are designed for soldering temperatures of about 1150° C. and contain from 5 to 15 mol % of Al2O3. Such high soldering temperatures are undesirable for modern fuel cells, since they subject the metallic substrate materials and other heat-sensitive materials to an excessive degree.
DE 10 2005 002 435 A1 describes composite solders which consist of an amorphous glass matrix and a crystalline phase. The glass matrix has high contents of CaO and MgO of greater than 20% by weight, but this leads to relatively high viscosities and high dielectric losses. Furthermore, the content of Al2O3 is at least 10% by weight, Al2O3 is usually used in a glass solder to control crystallization, but also reduces the thermal expansion of the solder and is therefore often counterproductive when glass solders are used for joining materials having a high thermal expansion.
DE 10122327 A1 describes a glass solder having a coefficient of thermal expansion α(20-300) of greater than 11×10−6 K−1 composed of the system BaO—CaO—SiO2 for joining ceramics and also metals in the high-temperature range. Particularly when joining materials having a coefficient of expansion α below 12×10−6 K−1, for example ZrO2 ceramics having a coefficient of thermal expansion α of 10×10−6 K−1, thermal stresses occur as a result of the poor match and these stresses can reduce the strength or even lead to complete failure of the joint. The glasses have a BaO content of up to 45-55% by weight. High BaO contents can lead to increased crystallization. Furthermore, the proportion of SiO2 is in a range from 35 to 45% by weight. Increasing SiO2 contents lead to a decrease in the thermal expansion and to an increase in the required joining temperature.